Fluid working machines include fluid-driven and/or fluid-driving machines, such as pumps, motors, and machines which can function as either a pump or as a motor in different operating modes.
When a fluid working machine operates as a pump, a low pressure manifold typically acts as a net source of fluid and a high pressure manifold typically acts as a net sink for fluid. When a fluid working machine operates as a motor, a high pressure manifold typically acts as a net source of fluid and a low pressure manifold typically acts as a net sink for fluid. Within this description and the appended claims, the terms “high pressure manifold” and “low pressure manifold” refer to manifolds with higher and lower pressures relative to each other. The pressure difference between the high and low pressure manifolds, and the absolute values of the pressure in the high and low pressure manifolds will depend on the application. For example, the pressure difference may be higher in the case of a pump which is optimised for a high power pumping application than in the case of a pump which is optimised to precisely determine the net displacement of fluid, for example, a pump for dispensing a metered amount of fluid (e.g. a liquid fuel), which may have only a minimal pressure difference between high and low pressure manifolds. A fluid working machine may have more than one low pressure manifold.
Although the invention will be illustrated with reference to applications in which the fluid is a liquid, such as a generally incompressible hydraulic liquid, the fluid could alternatively be a gas.
Fluid working machines are known which comprise a plurality of working chambers of cyclically varying volume, in which the displacement of fluid through the working chambers is regulated by electronically controllable valves, on a cycle by cycle basis and in phased relationship to cycles of working chamber volume, to determine the net throughput of fluid through the machine. For example, EP 0 361 927 disclosed a method of controlling the net throughput of fluid through a multi-chamber pump by opening and/or closing electronically controllable poppet valves, in phased relationship to cycles of working chamber volume, to regulate fluid communication between individual working chambers of the pump and a low pressure manifold. As a result, individual chambers are selectable by a controller, on a cycle by cycle basis, to either displace a predetermined fixed volume of fluid or to undergo an idle cycle with no net displacement of fluid, thereby enabling the net throughput of the pump to be matched dynamically to demand.
EP 0 494 236 developed this principle and included electronically controllable poppet valves which regulate fluid communication between individual working chambers and a high pressure manifold, thereby facilitating the provision of a fluid working machine functioning as either a pump or a motor in alternative operating modes. EP 1 537 333 introduced the possibility of part cycles, allowing individual cycles of individual working chambers to displace any of a plurality of different volumes of fluid to better match demand.
Key factors which determine the performance of fluid working machines of this type include the performance characteristics of the electronically controllable valves. These valves are typically electromagnetically actuated poppet valves, although other valves types could conceivably be employed. Relevant performance characteristics include the speed at which the electronically controllable valves open and close, the pressure difference against which they can open, their operational lifetime and the cross-section of the flow path through the valve whilst open, which limits the throughput of fluid and influences the flow characteristics of fluid into and out of the working chambers. Accordingly, the electronically controllable valves are an expensive and performance limiting component of such fluid working machines and it would be desirable to reduce one or more of the demands made on the electronically controllable valves.
In particular, a significant technical problem, which determines the specification of electronically controllable valves for a particular application, arises when fluid flows into a working chamber of a pump from a low pressure manifold during an expansion stroke of a working chamber. The rate of fluid flow is limited by the cross-section and geometry of the flow path through the poppet valve and the properties of the working fluid. Where the fluid flowing into the working chamber is a liquid, it is subject to cavitation, which increases noise, reduces efficiency by requiring a pressure difference across the poppet valve, and leads to damage to the machine. A different problem applies during the contraction stoke of a working chamber in a motor, when fluid flows out to a low pressure manifold, where an increased pressure drop causes inefficiency, and where the poppet valve may be inadvertently closed causing possible damage to the valve and inadvertent pumping.
This problem has typically been solved by specifying larger electronically controllable valves for higher throughput applications, or applications where superior fluid flow characteristics are required. However, larger electronically controllable valves are more expensive and there can be a trade off in performance characteristics. For example, larger electronically controllable valves may open and close more slowly than smaller valves or use more electrical power, forcing compromises to be made.
Accordingly, some aspects of the invention aim to reduce the performance demands on the electronically controllable valves, to facilitate improved performance or to enable smaller and/or reduced specification electronically controllable valves to be employed than would otherwise be the case to obtain a fluid working machine with specified performance characteristics. Some aspects of the invention also aim to reduce the build up of hot fluid that can occur in the crankcase in radial piston pumps and/or motors.
Further aspects of the invention address problems associated with opening the low pressure valve, which connects a working chamber to a low pressure manifold, in a fluid working motor (such as a fluid working machine which can function only as a motor, or a fluid working machine which can function either as a motor or a pump, in different operating modes). In a motoring cycle, a high pressure valve associated with the working chamber is closed, under the active control of the controller, shortly before the end of the expansion stroke. As the working chamber continues to expand, the pressure of the fluid trapped within the working chamber drops. Typically, the pressure of the fluid trapped within the working chamber will need to drop to close to the low pressure manifold pressure before the low pressure valve can open. However, it can take a significant period of time for the pressure of the fluid trapped within the working chamber to drop to a sufficiently low value, for several reasons. Firstly, the rate of change of working chamber volume decreases towards the end of the expansion stroke in most fluid working machines. Secondly, the variation in pressure of the fluid trapped within the working chamber is not a linear function of the volume of the working chamber, in the case of many commonly used hydraulic fluids. Furthermore, gases which are dissolved within the hydraulic fluid may evaporate, which has the effect of reducing the expected rate of decrease of pressure within the working chamber. This delay can reduce the efficiency of the fluid working motor. Indeed, malfunctions can arise if the pressure within the working chamber does not drop to a sufficiently low value to enable the opening of the low pressure valve, for example on start-up, or when operating in especially high or low temperature conditions.
Accordingly, some aspects of the invention aim to facilitate the opening of a low pressure valve, which regulates communication between the interior of a working chamber and a low pressure manifold, during a motoring cycle of a fluid working machine.